This invention relates generally to improved follower circuits and, more particularly, to an operational amplifier utilizing FET followers and feed-forward capacitors to improve frequency performance, slew rate, and enhance gain-bandwidth product (GBP).
It is well known that field-effect-transistor (FET) followers, in particular junction field-effect-transistor (JFET) followers have poor frequency response at low operating currents (e.g. .ltoreq.100 .mu.A) due to their low transconductance (g.sub.m). For example, in one known operational amplifier circuit, the base terminals of a PNP differential pair are each coupled to the source of a JFET follower. The high dynamic resistance (1/g.sub.m) of the JFET follower forms a pole with the equivalent input capacitance (C.sub..pi.) of the PNP transistor. The result is a JFET amplifier having poor frequency performance; e.g. 45 degrees of excess phase shift at 800 KHz. Notwithstanding, the use of JFETs as input devices is desirable since they require a significantly smaller input bias current.